SHRIMP zircon U-Pb dating for two episodes of migmatization in the Dabie orogen

Zircon CL imaging and SHRIMP U-Pb dating were carried out for migmatite in the Dabie orogen. Zircons from the Manshuihe migmatite show clear core-rim structures. The cores display sector or weak zoning and low Th/U ratios of 0.01 to 0.17, indicating their precipitation from metamorphic fluid. They yield a weighted mean age of 137±5 Ma. By contrast, the rims exhibit planar or nebulous zoning with relatively high Th/U ratios of 0.35 to 0.69, suggesting their growth from metamorphic melt. They give a weighted mean age of 124±2 Ma. Zircons from the Fenghuangguan migmatite also display core-rim structures. The cores are weakly oscillatory zoned or unzoned with high Th/U ratios of 0.21 to 3.03, representing inherited zircons of magmatic origin that experienced different degrees of solid-state recrystallization. SHRIMP U-Pb analyses obtain that its protolith was emplaced at 768±12 Ma, consistent with middle Neoproterozoic ages for protoliths of most UHP metaigneous rocks in the Dabie-Sulu orogenic belt. By contrast, the rims do not show significant zoning and have very low Th/U ratios of 0.01 to 0.09, typical of zircon crystallized from metamorphic fluid. They yield a weighted 206Pb/238U age of 137±4 Ma. Taking the two case dates together, it appears that there are two episodes of zircon growth and thus migmati-tization at 137±2 Ma and 124±2 Ma, respectively, due to metamorphic dehydration and partial melting. The appearance of metamorphic dehydration corresponds to the beginning of tectonic extension thus to the tectonic switch from crustal compression to extension in the Dabie orogen. On the other hand, the partial melting is responsible for the extensional climax, resulting in formation of coeval migmatite, granitoid and granulite. They share the common protolith, the collision-thickened continental crust of mid-Neoproterozoic ages.     

SHRIMP U-Pb zircon geochronology of Yangfang aegiriteaugite syenite in Wuyi Mountains of South China and its tectonic implications

In addition to the wide spread peraluminous granites, some alkaline or alkali-rich intrusive rocks were recently observed in the Wuyi Mountains. The Yangfang aegiriteaugite syenites in the Wuyi Mountains are strongly enriched in light rare earth elements (LREE), K, Ba, Ga, Zr and Y. What is more, differentiations of REE are obvious. In addition, the Yangfang aegiriteaugite syenites have also some characteristics similar to A-type granite. Zircons are selected from the Yangfang aegiriteaugite syenites for Sensitive High-Resolution Ion Microprobe (SHRIMP-Ⅱ) U-Pb geochronology study. 15 analyses give concordant 206Pb/238U ages in a narrow range of 233—249 Ma, which correspond to a single age population with a weighted mean 206Pb/238U age of 242 4 Ma, which records the crystallization time of the aegiriteaugite syenites. It is suggested that: (1) there were early Triassic peralkaline intrusive rocks in South China; (2) the early Indosinian extension event was probably active in local area, South China. This discovery provides a new insight for farther understanding of 揑ndosinian orogeny in South China.     

SHRIMP U-Pb zircon dating for Qiashikansayi granodiorite, the Northern Altyn Tagh Mountains and its geological implications

The Qiashikansayi granodiorite is foliated resulting in a granodioritic gneiss. Its geochemical features, such as alumina saturation index(A/CNK) of 0.81-0.99, Na2O/K2O values>1, TiO2 contents <1.0%, LREE enrichment with high fractionation factors, weakly negative or no Eu anomalies, and significant Ba and Ti negative anomalies, suggest that it is similar to a typical island arc pluton. The trace elements of the Qiashikansayi granodiorite are plotted in the island arc field in the tectonic setting discrimination diagrams as well. Cathodoluminescence images demonstrated that the zircons have clear rhythmic crystallized zoning, without any remnant core and new crystallized rim, suggesting the zircons be magmatic ones. Their U and Th contents vary in the range of 574-870 μg/g, and 279-556μg/g respectively, with the Th/U ratio in the range of 0.52-0.68. SHRIMP zircon U-Pb dating yielded the 481.5±5.3 Ma age for the intrusion of the granodiorite, which is coeval with the island arc volcanic rocks in the no     

甘肃北祁连武威地区莲花山花岗岩体的单颗粒锆石U-Pb年龄及其地质意义

在1:25万武威幅区域地质调查中对北祁连莲花山花岗岩体进行了调查研究,通过高精度单颗粒锆石U-Pb同位素测年,获得w(206Pb)/w(238U)表面年龄统计权重平均值为426±15 Ma,属加东晚期.岩石化学和地球化学特征表明莲花山岩体属钙碱性系列,少量为高钾钙碱性系列.σ=0.6~2.3,A/CNK=0.81~1.04,ΣREE=68.58~188.76,LREE/HREE=2.80~5.09,δEu=0.60~0.87.研究表明莲花山岩体形成于陆陆碰撞造山阶段后期,可能形成于加厚地壳的部分熔融.     

SHRIMP zircon U-Pb geochronology of Indosinian granites in Hunan Province and its petrogenetic implications

The structure, magmatism and sedimentation within the South China Block (SCB) related to the Indosinian Orogeny had attracted considerable attention since De-prat[1] and Fromagat[2] proposed the “Indosinian move-ment” based on two unconformities between Pre-Norian and Pre-Rhaetian during Triassic in Vietnam. However, Indosinian tectonic evolution of the SCB has been long debated[3―6]. Some researchers believed that the compli-cated structure-magmatism-sedimentation within the SCB w…     

SHRIMP zircon U-Pb dating for subduction-related granitic rocks in the northern part of east Junggar, Xinjiang

A great deal of studies have recently devoted to the Central Asian orogenic belt (CAOB). Some of the studies have proposed that CAOB is a tectonic frame ofcomplex mosaic fragments, link of multiple suture zone and mountain-basin coupling, and has undergon…     

党河南山乌里沟中酸性岩体锆石U-Pb年龄、地球化学特征及与金矿成矿关系

通过对南祁连山党河南山东段乌里沟金矿区中酸性侵入岩锆石U-Pb年龄测定及岩石地球化学分析,探讨了岩体形成的构造环境及其与金矿成矿的关系。矿区角闪石闪长岩体侵位年龄为457±6.3 Ma,岩石中各组分的质量分数:SiO2为48.98%~59.16%,Al2O3为14.51%~16.77%,K2O+Na2O为8.24%~9.47%,属准铝质、碱性-过碱性岩系列;DI为58~79,属I型花岗岩类;岩石Cr和Mg#值较低,Na2O和K2O含量接近,表明源岩含有较多的壳源成分;稀土总量中等,轻稀土富集,具弱Eu负异常;相对富集大离子亲石元素Rb、Ba、Th、K、U,亏损Nb、Ta、P、Ti。矿区二长花岗岩属碱性岩系列,比角闪石闪长岩稀土总量低、更加亏损Nb、Ta等,与角闪石闪长岩属同一岩浆源,分异程度比角闪石闪长岩高。两种岩石均具岛弧岩浆岩特征,是中晚奥陶世南祁连俯冲到较浅部位熔融形成。岩体金含量高,目前探明的矿体产于岩体内部及接触带,围岩蚀变强烈,显示岩体为金矿成矿提供物质来源和成矿热液。     

SHRIMP U-Pb geochronology of the zircons from the Precambrian basement of the Qilian Block and its geological significances

Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks. The results from this and previous studies indicate that the ages of the Precambrian detrital zircons from the Qilian Block are widely distributed in the Proterozoic era, distinct from the North China Block which was stable in the Neo-Mesoproterozoic era. By contrast, the age histograms of the detrital zircons from the Qilian Block is similar to those from Precambrian basement of the Yangtze Craton. Therefore, it is suggested that the Qilian Block had a strong affinity toward the Yangtze Craton and might belong to the supercontinent Gondwana in the Neoproterozoic time. This inference is supported by Nd model age (TDM), stratigraphic, and paleontological evidence. It is further considered that the Qilian Block was rifted from the supercontinent Gondwana during late Sinian to form an isolated continent in the Proto-Tethyan Ocean, moving towards the Alaxa Block in the North China Craton. The part of Proto-Tethyan Ocean between the Qilian and Alaxa Blocks should correspond to the so-called Paleo-Qilian Ocean. Following the closure of the Paleo-Qilian Ocean in the early Paleozoic, the Qilian Block collided with the Alaxa Block to form the North Qilian Orogenic Belt. Based on this tectonic explanation, the North Qilian ophiolites should represent parts of lithosphere from the Proto-Tethyan Ocean. Lithological and geochronological evidence also indicates that the Qilian Block underwent continental reactivation possibly induced by the deep northward subduction of the North Qaidam Block in early Paleozoic time.     

Triassic U-Pb age for zircon from granites in the Tonghua area and its response to the Dabie-Sulu ultrahigh-pressure collisional orogenesis

Single-grain zircon U-Pb dating was carried out to constrain the emplacement timing of granitic plutons at Chaxinzi, Xiaoweishahe and Longtou in the Tonghua area, south of Jilin Province. The results show that these plutons formed in the Triassic with ages of 203—217 Ma. Geological and geochemical characteristics indicate that the plutons are composed of quartz diorite and granite. The former was derived from partial melting of mafic lower crust, whereas the latter originated from thickened crust with garnet as the residue in the source. It appears that protoliths of these two types of granitits are different although they have the same emplacement age. Considering that these plutons are petrologically different from the coeval granites in the Xingmeng (Xing‘an-Mongolian) to Jihei (Jilin-Heilongjiang) orogenic belt in the north, it is suggested that their formation was related to the Dabie-Sulu ultrahigh-pressure collisional orogenesis since their ages are only 10—20 Ma younger than timing of the ultrahigh-pressure metamorphism, but comparable to that of the first rapid exhumation of the ultra-high-pressure metamorphic rocks and the emplacement of the post-collisional granites.     

SHRIMP U-Pb zircon dating for the dacite of the Sangxiu Formation in the central segment of Tethyan Himalaya and its implications

The Tethyan Himalaya is located between the In-dus-Yarlung Zangbo suture zone to the north and HigherHimalayan crystallines to the south, paleogeographicallybelonging to the northern margin of “Greater India”. TheSangxiu Formation is used to define a series of specialstrata with rich volcanic interbeds, and is only distributedto the southeast of Yangzuoyong Co within 200 km×50 km along the eastern part of central Tethyan Hima-laya[1]. Based on overlapped relationships of strata andfos…     

In situ simultaneous determination of trace elements,U-Pb and Lu-Hf isotopes in zircon and baddeleyite

This paper describes a combined method of simultaneously measuring U-Pb and Lu-Hf isotopes as well as trace elements in Phalaborwa baddeleyite and 91500, GJ-1, TEMORA-1 and SK10-2 zircons by means of Neptune MC-ICPMS and Agilent Q-ICPMS connected to a 193 nm excimer laser ablation system. Material ablated by laser was carried in different proportions into Q-ICPMS for U-Pb isotopic and trace elemental and MC-ICPMS for Lu-Hf isotopic compositions. Experiments indicate that different proportions of ablated material for the Q-ICPMS and MC-ICPMS (6:4, 5:5 and 4:6 respectively) do not show any bias for the zircon/baddeleyite U-Pb age, Lu-Hf isotope and trace elemental compositions within ana-lytical errors. Using 40―60 μm spot size, the obtained U-Pb ages of Phalaborwa baddeleyite, 91500, GJ-1, TEMORA and SK10-2 zircons are 2065±15 (2σ, n=20), 1063±6 (2σ, n=19), 613±6 (2σ, n=20), 416±5 (2σ, n=20) and 32.6±0.5 (2σ, n=20) Ma, respectively. The 176Hf/177Hf ratios are 0.281231±24 (2SD, n=20), 0.282310±35 (2SD, n=19), 0.282028±34 (2SD, n=20), 0.282687±34 (2SD, n=20) and 0.282752±53 (2SD, n=20), respectively. The obtained trace elemental compositions are identical to the reference values. Therefore, this kind of technique makes it possible to simultaneously obtain the U-Pb age, Lu-Hf iso-topes and trace elemental compositions of zircon and baddeleyite, which could be an important tool in solving problems in earth sciences.     

黑龙江金厂金矿花岗斑岩锆石地球化学、U-Pb年代学及地质意义

为确定与金厂金矿成矿有关的岩浆类型、活动时限和构造背景,采用LA-ICP-MS技术对研究区花岗斑岩开展了锆石U-Pb年龄及原位微区微量元素测定。结果表明:锆石环带发育,wTh/wU值>0.4,具有岩浆锆石特征;锆石年龄分布于220Ma和103～123Ma 2个区间,代表了2期岩浆事件;对2种锆石分别命名为捕获岩浆锆石和新生岩浆锆石;锆石的地球化学和年龄信息显示捕获岩浆锆石的原岩为早三叠世花岗岩;锆石微量元素信息暗示花岗斑岩是早三叠世花岗岩高度熔融结晶分异、侵位于浅部氧化环境而形成,这一过程导致新生岩浆锆石负Eu异常程度降低。新生锆石加权平均年龄为(113.5±3.8)Ma,与成矿年龄一致,据此认为早白垩世的岩浆事件是金厂金矿成矿事件的直接原因,成矿背景为太平洋板块俯冲后的岩石圈伸展。     

A combined study of SHRIMP U-Pb dating, trace element and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane

Methods recently advanced for discrimination on the genesis of metamorphic zircon, such as analysis of mineral inclusions and trace elements, provide us powerful means to distinguish zircon overgrowth during high-pressure metamorphism. Zircons in ultrahigh-pressure eclogite from Qinglongshan in the Sulu terrane were studied by the SHRIMP U-Pb method in combining with trace element and mineral inclusion analyses. No inherited core was identified in the analyzed zircons by means of cathodoluminescence images. The occurrence of high-pressure metamorphic mineral inclusions in zircon, such as garnet, omphacite, rutile, and the flat HREE pattern in zircon indicate that the zircon formed at high-pressure metamorphic conditions. Therefore, a weighted average U-Pb age of 227.4±3.5 Ma obtained from such a kind of zircon is interpreted to represent the timing of peak nietamorphism for the Qinglongshan eclogite.     

Qinghu zircon: A working reference for microbeam analysis of U-Pb age and Hf and O isotopes

Zircon is the most useful mineral for studies in U-Pb geochronology and Hf and O isotope geochemistry. Matrix effect is a major problem of the microbeam techniques such as SIMS and LA-（MC）-ICPMS. Therefore, external standardization using well-characterized natural zircon standards is fundamental for accurate microbeam measurements. While the isotopic geochro- nology and geochemistry laboratories equipped with microbeam analytical facilities have been increasingly established in China during the past decade, applications of the isotopic microanalysis are still limited due to shortage of available standards. We report here the Qinghu zircon as a potential new working reference for microbeam analysis of zircon U-Pb age and O-Hf isotopes. This zircon was separated from the Qinghu quartz monzonite from the western Nanling Range, Southeast China. It is fairly homoge- neous in U-Pb age and Hf and O isotopes in terms of large amounts of mircobeam measurements by LA-MC-ICPMS and SIMS at the scales of 20-60 ~tm. SIMS measurements yield consistent 2~6pb/238U age within analytical uncertainties with that obtained by ID-TIMS. Precise determinations of O isotopes by IRMS and Hf isotopes by solution MC-ICPMS are in good agreement with the statistical mean of microbeam measurements. We recommend U-Pb age of = 159.5 ± 0.2 Ma （2SE）, δ^18O = 5.4‰ ± 0.2%0 （2SD） and 176Hf/177Hf = 0.283002 ± 0.000004 （2SD） as the best reference values for the Qinghu zircon.     

西藏冈底斯西部地区花岗岩锆石U-Pb年龄及其地质意义

对西藏冈底斯西部措勤县打加错地区粗粒二长花岗岩、中粒二长花岗岩、花岗岩进行LA-ICP-MS锆石U-Pb法定年、锆石同位素组成、主量元素测定,获得年龄值分别为(43.5±1.5)Ma、(44.2±2.6)Ma及(44.2±1.7)Ma,结合区域岩浆侵位规律,根据侵入岩地质特征、岩石学、同位素年代学、岩石化学等特征,将研究区出露的花岗岩划分为喜山早期侵入岩,属冈底斯-念青唐古拉中酸性杂岩带南亚带重要组成部分之一。其物质来源与上地壳变质泥岩的部分熔融有关,所形成的构造环境与区域构造背景息息相关,是始新世末喜马拉雅陆块同冈底斯陆块碰撞,陆-陆碰撞造山结束后相对松弛阶段的产物,构造环境同碰撞花岗岩。     

U-Pb zircon age for the Daohugou Biota at Ningcheng of Inner Mongolia and comments on related issues

SHRIMP U-Pb zircon dating was carried out for the Daohugou Biota near Ningcheng of Inner Mongolia and for lavas overlying or underlying sala-mander-bearing strata at Reshuitang in Lingyuan of West Liaoning. The results suggest that the Dao- hugou Biota occurred at an interval from 168 Ma to 164―152 Ma. Both the Daohugou Biota and the salamander-bearing fossil assemblage are the same biota and thus developed from 168 to 152 Ma, i.e. from late Middle Jurassic to the early Late Jurassic. The Daohugou Biota-bearing rocks, resting on the Jiulongshan Formation in disconformity and being overlain in unconformity by Late Jurassic Tuchengzi Formation and Early Cretaceous rocks containing the Jehol Biota, are mainly composed of volcanic-sedi- mentary rocks in a normal sequence. It is recom- mended that the Daohugou Biota and the related stratigraphy should be correlated with the Tiaojishan Formation (Lanqi Formation in West Liaoning) or its synchronous rocks. It is suggested that the Dao- hugou Biota and the Jehol Biota would be neither taken into one biota nor considered as the earliest elements of the Jehol Biota. The Daohugou Biota and the related rocks and the Yixian Formation were respectively formed in different periods of volcanic-sedimentary tectonics.     

江西南部版石盆地火山岩SHRIMP锆石U-Pb年龄及其地质意义

应用SHRIMP锆石U-Pb定年方法,对江西南部三南-寻乌火山岩带版石盆地原鸡笼嶂组凝灰岩和原版石组流纹岩开展年代学研究.结果表明,凝灰岩SHRIMP锆石U-Pb年龄为(142.5±1.3)Ma,流纹岩SHRIMP锆石U-Pb年龄为(131.4±1.3)Ma.根据新的国际地层表,晚侏罗世与早白垩世的界线划在(145.5...     

Emplacement age of the Tahe granite and its constraints on the tectonic nature of the Ergun block in the northern part of the Da Hinggan Range

Large-scale granitic plutons are exposed in the Ergun block in the northern part of the Da Hinggan Range, but their emplacement age and petrogenesis remain unknown. Of these plutons, the Tahe pluton is composed mainly of porphyritic syenogranite and monzogranite, with minor hornblende alkali feldspar granite and gabbro, which have affinities to post-orogenic granitoids. Laser-ablation Inductively Coupled Plasma Mass-spectroscope （LA-ICP-MS） analysis shows that the major rock types of the Tahe pluton formed at 494- 480 Ma ago, indicating its emplacement in the Early Paleozoic. It is concluded, therefore, that the collision between the Ergun and the Xing＇an blocks ended in Early Paleozoic. Considering the geochronological data of the plutons in the adjacent areas, the Early Paleozoic evolutionary history of the Ergun block is similar to the central Mongolia and the Tuvino blocks in Mongolia, and the Ergun block should be a part of the accretionary continental margin in the southern Siberian Craton.